High electron mobility transistors (HEMTs) based on a hetero-structure of two Group III-V compound semiconductors exhibit excellent two dimensional electron gas (2DEG) characteristics due to its large band offset and polarization induced charge at the hetero-interface, resulting in a high sheet concentration and a high mobility. If the Group III-V semiconductor layers are formed by MOCVD (Metal-Organic Chemical Vapor Deposition), high growth temperatures are generally required. Besides, the step coverage of the thin films prepared by MOCVD is poor when deposited on a structure with a high aspect ratio, such as a fin structure. Accordingly, it is generally difficult to grow Group III-V compound semiconductor hetero-structures by MOCVD on side walls of a nano-scale fin. The poor interface quality at the sidewalls may induce strong electron scattering and reduce the device mobility.